Fabry-perot etalon with independently selectable resonance frequency and free spectral range

ABSTRACT

The invention relates generally to optical interference filters and interferometers. Methods, devices and device components for optical signal generation and processing using optical interference filters and interferometers are presented. The invention provides optical interference filters and interferometers having a selected cumulative reflectance phase dispersion capable of providing substantially independent selectable resonance frequency and free spectral range. An exemplary interference filter of the present invention provides a multi-peak transmission spectrum with substantially independent, selectable control over absolute transmission band frequencies and relative transmission band spacing. The methods and devices provided herein are particularly well suited for frequency matching optical signals to a selected frequency standard, such as the International Telecommunication Union frequency standard.

FIELD OF INVENTION

[0001] This invention relates generally to optical interference filtersand devices and device components employing optical interferencefilters. In particular, the invention relates to Fabry-Perot opticalinterference filters with a selected cumulative reflectance phasedispersion capable of providing substantially independent, selectableresonance frequency and free spectral range, which are useful forfrequency matching optical signals to a particular frequency standard.

BACKGROUND OF INVENTION

[0002] The Fabry-Perot (FP) interferometer has played a seminal role inthe development of a number of scientific fields including atomic andmolecular spectroscopy, material science, astronomy, lasers and opticalcommunications. Fabry-Perot etalons are optical filters based on the FPinterferometer. These filter are capable of providing extremely highresolution approaching tens of megaHertz at optical frequencies, veryhigh light throughput and can comprise active optical components capableof fine wavelength tuning.

[0003] The FP etalon optical filter operates by multiple-beaminterference of light reflected and transmitted by a pair of parallel,flat optical reflectors. In its most basic configuration, a FP etalonfilter comprises an optical resonance cavity formed between twopartially reflective, low loss reflectors. Typically, the resonancecavity is an air gap or a dielectric material having an optical pathlength equal to an integer multiple of one half the wavelength of lightto be transmitted by the filter and the reflectors comprise dielectricstacks of alternating high and low refractive index layers. Such FPetalon designs have been used in both pulsed and continuous wave opticalfiltering applications and have been demonstrated to effectively filterlight over the ultraviolet, visible and infrared spectral regions.

[0004] Light propagating through the FP etalon is partially transmittedand reflected upon every interaction with each reflector. The paralleloptical configuration of the FP etalon provides for multiplereflections, which results in constructive and/or destructiveinterference depending on the wavelength of light propagating throughthe cavity. As a result of optical interference of reflected andtransmitted beams, only certain frequencies of light are transmitted bythe FP etalon filter. The transmitted frequencies correspond to theresonance frequencies of the etalon filter. Therefore, etalontransmission spectra typically comprise a plurality of transmissionbands that are evenly separated from one another by the free spectralrange of the etalon filter. The resonance frequency, free spectralrange, light throughput and band width of transmission bands depend onseveral parameters including: (1) the optical path length of theresonance cavity, (2) the index of refraction and index dispersion ofthe cavity (3) reflectance of the reflectors and (4) extent ofparallelism of the pair of reflectors. FP etalon filters of this typeare extensively described in by Moore et al. in “Building ScientificApparatus”, Addison-Wesley Publishing Co, 1989, pgs. 242-251 and Hect in“Optics, 2^(nd) Edition”, Addison-Wesley Publishing Co, 1987, pgs.368-372.

[0005] In long-haul telecommunication systems, optical signals are oftengenerated from electronic signals, transported great distances alongoptical fiber networks and detected in a manner to regenerate theoriginal electronic signal. While such telecommunication systems exploitthe substantial efficiency gains of using optical methods for signaltransmission, signal processing via standard electronic techniquesremains a barrier to achieving the highest overall efficiency andaccuracy of the telecommunication network. Specifically, the use ofelectronic components for signal processing often imposes substantialphysical limitations on the speed that information can be transmittedand utilized. Accordingly, to fully realize the efficiency and accuracypossible in a purely optical telecommunication system, a need currentlyexists for optical communication technology capable of direct opticalprocessing of telecommunication signals. Examples of such optical signalgeneration and processing applications include wavelength stabilizationtechniques, dispersion compensation methods and wavelength multiplexingtechniques and are described in detail in U.S. Pat. Nos. 5,798,859,6,208,444, 6,169,626 and 5,999,320.

[0006] Improvements in purely optical signal processing have focused ondevelopment of optical devices capable of all aspects of signalgeneration, processing and detection. These efforts include researchinto new optical devices that perform a variety of signal transmissionand processing functions including signal amplification, beam splitting,signal coupling, optical filtering, multiplexing, demultiplexing opticalswitching and dispersion correction. High throughput optical filterswith selectable transmission frequencies and bandwidth are essentialcomponents of a wide variety of such optical devices. FP etalon filterscapable of providing these functions are of great importance to thedevelopment of highly efficient and accurate optical telecommunicationsystems.

[0007] Wavelength division multiplexing is used to increase thetransmission capacity of fiber optic communication systems by allowingmultiple wavelengths to be transmitted and received over a singleoptical fiber. In wavelength division multiplexing, a plurality ofoptical signals of different wavelength are multiplexed by coupling eachsignal to a common transmission line. The multiplexed transmissionsignal is then propagated over a single optical medium to a variety ofreceivers. When received, the (multiplexed transmission signal isdemultiplexed into separate channels corresponding to individualwavelengths and detected by a receiver. Typically, signal demultiplexingis achieved by a variety of wavelength selective optical filteringdevices including optical interference filters, cutoff filters, prismsand diffraction gratings. Although wavelength division multiplexingprovides a simple, effective and low cost way of increasing transmissioncapacity, the feasibility of this technology is dependent on thedevelopment of high resolution, high throughput filters and transmittinglasers with highly stable and accurate frequencies.

[0008] Adoption of universal standard transmission channels for fiberoptic transmission promotes efficient application of wavelength divisionmultiplexing. The International Telecommunication Union (ITU) hasadopted a standard channel definition providing a 45 channel system overa wavelength range of 1533 nm to 1565 nm with a uniform channel spacingof 100 GHz (approximately 0.8 nm). High resolution, high throughputoptical filters capable of acting as a reference for the ITU standardsare needed in the art.

[0009] The high throughput nature of FP etalon filters makes their useas optical frequency discriminators in devices that generate, detect orprocess optical signals especially attractive. While the periodic natureof the transmission spectra of FP etalon filters makes them ideallysuited for wavelength discrimination in multiplex applications employingequally spaced transmission channels, their use is currently hampered byfundamental spectral limitations impeding accurate frequency matching ofthe transmission bands of a standard FP etalon filter with thetransmission channels defined by the ITU frequency standard. Thesespectral limitations include the inability of FP etalon filters of theprior art to have the appropriate free spectral range and resonancefrequency to overlap the transmission channels of the ITU frequencystandard.

[0010] Both the free spectral range (Δν) of a FP optical filter and theresonance frequency (ν).depend on the optical path length (L) of theresonance cavity and the cumulative reflectance phase dispersion of thereflectors. As a result, most FP etalon designs currently available donot provide independent and selectable control of free spectral rangeand resonance frequency. Rather, etalons in the prior art only provideindependent selection of either one of these two variables.Unfortunately, these designs do not provide FP etalon filters withtransmission spectra that match the channel spacing and position of theuniversally adopted ITU frequency standard.

[0011] The ability to selectively and independently control theresonance frequency and free spectral range of an etalon optical filterwould aid tremendously in realizing the full potential of wavelengthdivision multiplexing in optical telecommunication systems. First, FPetalon filters with transmission spectra selected to match the adoptedtransmission channels of the ITU frequency standard would provide auniversally applicable frequency reference for distributed feedbacklasers, which comprise an important optical source fortelecommunications signaling. Specifically, these filters would possessthe high resolution needed for laser frequency monitoring and control atall transmission frequencies of the ITU transmission channels, withinthe narrow tolerances needed for efficient signal multiplexing, whichcan approach 10-20 ppm.

[0012] Second, FP etalon filters with independently adjustable resonancefrequency and free spectral range would provide frequency discriminatorsideally suited for signal demultiplexing applications. In particular,these filters would allow selectable, high throughput transmission oflight corresponding to one or more transmission channels in the ITUfrequency grid. Such etalon optical filters would provide accuratewavelength discrimination and signal processing with minimal loss ofsignal.

[0013] Finally, FP etalon filters with independently adjustableresonance frequency and free spectral range would provide importantmeans for correcting chromatic dispersion inherent, to wavelengthmultiplex signals that propagate over long fiber distances. Chromaticdispersion is caused by the dependence of the refractive index of silicaon wavelength and causes different parts of the signal spectrum toarrive at the distant end of the system at different times. FP etalonfilters can be used to compensate for chromatic dispersion because theoptical frequency of any portion of the signal contains the informationof the delay that has occurred.

[0014] U.S. Pat. No. 5,212,584 discloses a tunable etalon filter forwavelength division multiplex optical communication systems.Specifically, the etalon design disclosed is reported to provideselectable control of resonance frequency by employing a temperaturecontrolled resonance cavity comprising a spacer composed of a materialwith a relatively large rate of change of refractive index withtemperature. While the resonance frequency is reported to varysystematically with angle of incidence and cavity temperature, theetalon described in U.S. Pat. No. 5,212,584 does not providesubstantially independent selectable resonance frequency and freespectral range. Therefore, selection of the resonance frequency of theetalon fixes the free spectral range to a set value. Accordingly, U.S.Pat. No. 5,212, 584 does not disclose methods of frequency matching thetransmission bands of an etalon optical filter with the plurality oftransmission channels or emission lines of a given frequency standard,such as the ITU frequency grid.

[0015] U.S. Pat. No. 5,291,332 discloses FP etalon designs havingselected reflectance phase dispersion characteristics, which arereported to match aperiodic atmospheric spectral lines. The FP etalondesign described employs reflectors having rugate coatings selected toachieve a prescribed reflectance phase dispersion. Specifically, thephase and frequency of the sinusoidal index of refraction profile of therugate coating is selected to achieve the desired FP etalon transmissioncharacteristics. While U.S. Pat. No. 5,291,332 reports successfulfrequency matching of the etalon transmission spectra and aperiodicatmospheric spectral lines, the reference does not disclose methods offrequency matching periodic spectral lines. Particularly, the referencedoes not disclose or suggest devices or methods capable of independentlyadjusting etalon resonance frequency and free spectral range whilepreserving a substantially periodic transmission spectrum. Further, thereference does not disclose techniques for frequency matching etalontransmission spectra and the evenly spaced transmission lines of givenfrequency standard or the evenly spaced lines of an optical source.Finally, the etalon design described in U.S. Pat. No. 5,291,332 employscostly rugate reflectors that are difficult to manufacture.

[0016] U.S. Pat. No. 6,154,318 discloses dispersive multilayer mirrorstructures that reportedly provide selectable reflectance group delay.Specifically, the authors report the use of a multilayer sequence ofthin dielectric films to provide selectable adjustment of thereflectance group delay by using multiple resonance trapping techniques.Although the mirror structures disclosed are reported to providereflection at selected frequencies, U.S. Pat. No. 6,154,318 does notdisclose optical methods for frequency filtering employing Fabry-Perotinterferometry. Further, the methods disclosed are limited to the use ofmultilayer mirror structures for pulsed optical source applications.Finally, the methods and devices disclosed by the authors to achieve thedesired resonance trapping properties are limited to “resonantsubstructures arranged around layers less than one quarter-wavelengthoptical thickness.”

[0017] It will be appreciated from the foregoing that a need exists forFP optical filters with independently selectable resonance frequency andfree spectral range. The present invention provides high throughput FPetalon filters able to provide substantially independent selection ofboth resonance frequency and free spectral range. Further, the presentinvention provides FP etalon structures that are capable of frequencymatching optical signals to the transmission channels of any selectedfrequency standard, particularly the International TelecommunicationsUnion frequency standard.

SUMMARY OF THE INVENTION

[0018] This invention provides methods, devices, and device componentsfor improving frequency discrimination and optical signal processingusing optical interference filters and interferometers. In particular,the invention provides for optical interference filters andinterferometers with selectable and adjustable cumulative reflectancephase dispersion. More specifically, this invention provides opticalinterference filters and interferometers having a multi-peaktransmission profile with substantially independent, selectable controlover absolute transmission band frequencies and relative transmissionband spacing. Also provided are optical interference filters andinterferometers, which are frequency matched to a selected frequencystandard, particularly the International Telecommunication Unionfrequency standard.

[0019] An optical interference filter of the present invention comprisesa FP etalon filter with substantially independent, selectable control ofthe resonance frequency and free spectral range. In this embodiment, theFP etalon filter has a selected cumulative reflectance phase dispersion,which provides the desired transmission characteristics. Specifically,selection of the cumulative reflectance phase dispersion providesaccurate control of the resonance frequency and the free spectral rangeof the FP etalon filter. As result of this functionally, an FP etalonfilter of the present invention is capable of providing a multi-peakedtransmission spectrum wherein the frequencies of transmission bands andthe relative spacing between transmission bands are substantiallyindependently selectable. In a preferred embodiment, the FP etalonfilters of the present invention are capable of providing substantiallyperiodic transmission bands. Specifically, this embodiment provides areflection phase shift at each reflector that is substantially linearwith respect to the frequency of incident radiation. Substantiallinearity of the reflection phase shift with respect to frequency isachieved by providing reflectors that exhibit substantially constantreflectance as a function of frequency. Optical filters havingperiodically spaced transmission bands are beneficial because they maybe frequency matched with the evenly spaced channels of a frequencystandard or evenly spaced emission lines of a given optical source.

[0020] Alternatively, FP etalon filters of the present invention arecapable of providing nonuniform, aperiodically spaced transmissionbands. In this embodiment, the cumulative reflection phase shift at eachreflector is substantially nonlinear with respect to frequency.Accordingly, the precise nonlinear dependency of the reflection phaseshift with frequency is chosen to provide control over the frequency andspacing of each transmission band. Optical filters with aperiodictransmission bands are beneficial because they may be frequency matchedto a frequency standard having unevenly spaced transmission channels orthe unevenly spaced spectral lines of a light emitting source,particularly with various modes of a laser. Further, optical filterswith aperiodic transmission bands are beneficial because they arecapable of passing some, but not all, transmission frequencies of agiven evenly spaced frequency standard.

[0021] In an exemplary embodiment, an optical interference filter of thepresent invention comprises a first partially reflective reflector and asecond partially reflective reflector in optical communication with eachother. The reflectors are positioned to intersect a propagation axis andare located in substantially parallel planes with respect to oneanother. The pair of reflectors form a resonance cavity along thepropagation axis between the first and second reflector with aselectably, adjustable optical path length. In a preferred embodiment,the first reflector comprises a first sequence of thin dielectric layerscomprising alternating high and low indices of refraction layers and anabsentee layer. The second reflector comprises a second sequence of thindielectric layers comprising alternating high and low indices ofrefraction layers. In this embodiment, optical thickness and compositionof the absentee layer and the position of the absentee layer in thefirst sequence of thin dielectric layers is selectably adjustable toprovide a selected cumulative reflectance phase dispersion of the FPetalon filter. In a preferred embodiment, the cumulative reflectancephase dispersion is selected from the range of about 1.0×10³¹ ⁵ rad/GHzto about 1.0×10⁻³ rad/GHz. In a more preferred embodiment, thecumulative reflectance phase dispersion is selected from the range ofabout 1.0×10⁻⁵ rad/GHz to about 1.0×10⁻⁴ rad/GHz. Particularly, thecumulative reflectance phase dispersion is selected to provideadjustable, substantially independent control over resonance frequencyand free spectral range of the FP etalon filter. Accordingly, thepresent invention includes FP etalon filters in which the frequency ofthe transmission bands is selectable.

[0022] Alternatively, the first reflector of the present invention maycomprise a sequence of thin dielectric layers comprising alternatinghigh and low indices of refraction layers and a plurality of absenteelayers. The number and composition of absentee layers and theirpositions in the sequence of thin dielectric layers is selectablyadjustable to provide a selected cumulative reflectance phase dispersionof the FP etalon filter. The use of a plurality of absentee layers inthe first sequence is beneficial because it provides more flexible andmore continuous control of the cumulative reflectance phase dispersionof the optical filter. More flexible and smoother control of thecumulative reflectance phase dispersion results in a higher degree ofindependent control over the resonance frequency and free spectralrange. Independent control of the position of the resonance frequencyand transmission band spacing is beneficial because it aids considerablyin precisely frequency matching the transmission bands of an opticalfilter to a given frequency standard.

[0023] In another embodiment, the second reflector of the presentinvention comprises a sequence of thin dielectric layers comprisingalternating high and low indices of refraction layers and one or moreabsentee layers. The number and composition of absentee layers andposition of one or more absentee layers in the sequence of thindielectric layers in the second reflector may be selected so that thesecond reflector has the same reflectance phase dispersion as the firstreflector.

[0024] Alternatively, the second reflector may have a selectedreflectance phase dispersion different from the first reflector. Use ofa second reflector comprising absentee layers provides even greatercontrol of the cumulative reflectance phase dispersion. Such flexibleand smooth control of cumulative reflectance phase dispersion isbeneficial because it provides greater control over the transmissioncharacteristics of the etalon filter, namely substantially independentcontrol of resonance frequency and free spectral range.

[0025] The interference filter of the present invention operates bymultiple-beam interference of light reflected and transmitted by firstand second partially reflective reflectors. Specifically, an incidentlight beam directed on to the optical interference filter undergoespartial reflection and partial transmission upon each interaction withthe alternating high and low indices of layers comprising the first andsecond reflectors. The discrete beams formed undergo constructive ordestructive interference as they translate through the optical filter.The nature and extent of the optical interference depends strongly onwavelength, resulting in a transmission profile characterized by avariety of transmission bands. The transmission bands of the etalonfilter of the present invention may be periodic, wherein the bands areevenly spaced with respect to frequency. Alternatively, the transmissionbands of the etalon filter of the present invention may be aperiodic,wherein the bands are unevenly spaced with respect to frequency.

[0026] In addition to interacting with the high and low indices ofrefraction layers, light propagating through the interference filter ofthe present invention also substantially interacts with at least oneabsentee layer. The presence of one or more absentee layers in thesequence of dielectric layers comprising the first reflector, secondreflector or both does not substantially affect the extent ofreflectance. The presence of one or more absentee layers does, however,significantly increase the cumulative reflectance phase dispersion ofthe etalon filter. Selection of the appropriate cumulative reflectancephase dispersion substantially affects the transmission characteristicsof the optical filter, namely the free spectral range and the resonancefrequency. Accordingly, the present invention includes FP etalon filterswith a selected cumulative reflectance phase dispersion that results inselection and control of the etalon transmission characteristics.

[0027] The free spectral range of a FP optical filter comprising twoidentical cavity reflectors and with a reflection phase shift relativeto the physical surface of the reflector of π is determined by theexpression: $\begin{matrix}{{\Delta \quad v} = \frac{c}{{2{nL}} + {\frac{c}{2\pi}\left( {\alpha_{1} + \alpha_{2}} \right)}}} & I\end{matrix}$

[0028] where Δν is the free spectral range, c is the speed of light in avacuum, L is the optical path length of the resonance cavity, n is therefractive index, α₁ is the reflectance phase dispersion of the firstreflector and α₂ is the reflectance phase dispersion of the secondreflector. The cumulative reflectance phase dispersion is defined as thesum of the reflectance phase dispersion of first and second reflectors:$\begin{matrix}{v_{m} = {\frac{c}{{2{nL}} + {\frac{c}{2\pi}\left( {\alpha_{1} + \alpha_{2}} \right)}}\left( {m - \frac{{\Phi_{1}\left( v_{c} \right)} - {\alpha_{1}v_{c}}}{2\pi} - \frac{{\Phi_{2}\left( v_{c} \right)} - {\alpha_{2}v_{c}}}{2\pi}} \right)}} & {III}\end{matrix}$

[0029] The resonance frequency of such a FP etalon filter also dependson the optical path length of the resonance cavity and the cumulativereflectance phase dispersion and may be expressed by the equation:$\begin{matrix}{\alpha_{cumulative} = {\alpha_{1} + \alpha_{2}}} & {II}\end{matrix}$

[0030] where ν_(m) is the resonance frequency, m is the order, Φ₁ is thereflection phase shift of the first reflector, and Φ₂ is the reflectionphase shift of the second reflector. ν_(c) is the center resonancefrequency of each reflector, which may be selected from a distributionof etalon resonance frequencies positioned about the reflectance maximumof either reflector. Φ(ν_(c)) is equal to π for reflectors comprising analternative sequence of high refractive index and low refractive index ¼wave layers and diverges from a value of π for non-¼ wave thindielectric films.

[0031] Analytically, the system may be considered a relationship betweentwo independent variables, Δν and ν_(m) and two dependent variables, Land α_(cumulative). Although Δν and ν_(m) are coupled due to thedependence of each on both dependent variables, Δν exhibits a muchstronger dependence on α_(cumulative) than does ν_(m). This is due tothe presence of the phase dispersion terms (α₁ and α₂) in the numeratorof equation III. Therefore, in practice Δν and Δ_(m) are substantiallyindependent variables. Accordingly, the resonance frequency and freespectral range of the FP etalon of the present invention may beindependently manipulated by the proper selection of the cumulativereflectance phase dispersion and resonance cavity length. The ability tocontrol the cumulative reflectance phase dispersion via the presence ofabsentee layers in the thin film sequences comprising first and/orsecond reflectors provides an accurate means of independently adjustingthe resonance frequency and free spectral range of the opticalinterference filter of the present invention.

[0032] The presence of one or more absentee layer increases thereflectance phase dispersion of the reflector, and, thus, determines thecumulative reflectance phase dispersion of the FP etalon filter. Themagnitude of the increase in reflectance phase dispersion iscontrollable by varying (1) the number of absentee layers present in thesequence of thin dielectric layers, (2) the position of absentee layersin either sequence of thin dielectric layers, (3) the composition of oneor more absentee layers and (4) the refractive index of one or moreabsentee layers. In practice, the number, optical thickness, compositionand position of absentee layers required to achieve selected cumulativereflectance phase dispersion are determined empirically. Alternatively,the number, optical thickness, composition and position of absenteelayer necessary to achieve a selected cumulative reflectance phasedispersion may be determined by numerical modeling methods. Suchnumerical modeling methods are well known in the art, such a TheEssential Macleod, software written by Thin Film Center.

[0033] The optical interference filter of the present invention may beused with continuous and/or pulsed light sources. Light directed at theoptical interference filter of the present invention may be oriented atan angle of incidence substantially normal to the plane containing thefirst partially reflective reflector. Alternatively, non-normal anglesof incidence may be employed. Varying in the angle of incidence of lightdirected at the optical interference filter of the present invention isuseful because it may substantially change the optical path length ofthe resonance cavity. Accordingly, the transmission properties of theetalon filter of the present invention may be tuned by variation of theangle of incidence of the incident light beam.

[0034] The first and second reflectors in the present invention may havethe same or different reflectivities. Reflectivities useful for thepresent invention range from 0.05% to 100%. High reflectivity reflectorsare beneficial because they provide optical interference filters withvery high finesse, useful for high-resolution applications.Alternatively, low reflectivity reflectors are beneficial because theytypically provide a more desirable discriminator slope for edge-lockingapplications. The first and second reflectors in the present inventionmay be composed of any combination of dielectric layers that exhibits atleast partial reflectivity. In a preferred embodiment, the dielectriclayers comprise thin dielectric films with alternative high and lowrefractive indices. In an exemplary preferred embodiment, the high orlow index of refraction layers comprise thin metal oxide layers,including but not limited to Ta₂O₅, SiO₂, HfO₂, MgF₂, CaF₂, TiO₂ andNb₂O₅. In addition, the first and second reflectors of the presentinvention may also have antireflection coatings on their exterior ends.Use of antireflection coatings is beneficial because it eliminatesunwanted reflections and results in higher light throughput. Finally,the reflectors of the present invention may be any size or shapeincluding but not limited to substantially wedged shape reflectors.

[0035] The resonance cavity of the present invention may be composed ofany dielectric material. In an exemplary embodiment, the resonancecavity is a dielectric cavity layer of a selected optical thickness.Alternatively, an optical interference filter of the present inventioncomprises an air gap resonance cavity of selected optical path length,wherein the space between first and second filters is occupied by aselected pressure of one or more gases or by a substantial vacuum. Inthis embodiment, an air gap alignment spacer or kinematic mountingsystem is necessary to maintain a substantially constant optical pathlength through the resonance cavity for a given angle of incidence. Airgap resonance cavities are beneficial because they provide opticalinterference filters that are thermally stable. Thermal stability isdesirable because it provides for very stable etalon transmissioncharacteristics, namely resonance frequency and free spectral range. Inaddition, air gap resonance cavities are beneficial because they providetunable etalon filters, wherein the refractive index and opticalthickness of the cavity can be selectably adjusted by varying thepartial pressure and identity of one or more gases in the cavity.

[0036] For a given angle of incidence, resonance cavities of the presentinvention may be of a substantially fixed, selected optical path lengthor may be of selectably, variable optical path length. Resonancecavities with a fixed optical path length may be beneficial because theyare capable of providing a very stable optical path length for a givenangle of incidence, and, thus provide very reproducible transmissionspectra. Resonance cavities with a variable optical path length arebeneficial because they are capable of providing tunable transmissioncharacteristics. Specifically, interference filters of the presentinvention with a variable optical path length resonance cavity arecapable of selectably adjusting the resonance frequency by variation ofthe optical path length.

[0037] Optical interference filters of the present invention may be usedindividually for frequency discrimination applications, such aswavelength division multiplexing and demultiplexing applications.Alternatively, series may be employed comprising a plurality of opticalfilters of the present invention in optical communication and positionedto intersect a common propagation axis. In an exemplary embodiment, FPfilters of the present invention are aligned in series. The use of aplurality of interference filters is desirable because it may providevery high-resolution frequency discrimination.

[0038] The optical interference filters and interferometers of thepresent invention are capable of frequency matching an optical signal toa selected frequency standard. In a preferred embodiment, an opticalinterference filter of the present invention comprises a FP etalonfilter having a well-characterized transmission spectrum comprising aplurality of transmission bands that substantially overlap one or moreof the transmission channels and/or spectral lines of a given frequencystandard. In another preferred embodiment, the etalon filter of thepresent invention has a transmission spectrum that is adjustable to passonly selected transmission channels and/or spectral lines of a givenfrequency standard. For example, a FP etalon filter of the presentinvention has a transmission spectrum that overlaps one or more selectedtransmission channels of the International Telecommunication frequencystandard. In a preferred embodiment, the intersection of the frequencystandard and transmission band occurs in a highly sloped region of thetransmission band. Alternatively, a FP etalon filter of the presentinvention has a transmission spectrum that overlaps one or more spectrallines of a light emitting source, such as a laser or radiating body.

[0039] The present invention also comprises methods and devices formonitoring and tuning the frequency of optical sources. In thisembodiment, incident light from an optical source is directed on a beamsplitter and the reflected light is passed through an opticalinterference filter having a substantially independent selectableresonance frequency and free spectral range. As set forth above, theoptical interference filter comprises a first and second reflector. Thefirst reflector has an internal and external end and comprises a firstsequence of thin dielectric layers comprising alternating high and lowindices of refraction layers and at least one absentee layer. The secondreflector is positioned a selected distance from the internal end of thefirst reflector and comprises a second sequence of thin dielectriclayers comprising alternating high and low indices of refraction layers.Both reflectors are located in substantially parallel planes withrespect to one another and form a resonance cavity there between. Thenumber of absentee layers, their composition and their position in thefirst and second sequence of thin dielectric films is selected toprovide a selected cumulative reflectance phase dispersion, whichdetermines the precise transmission properties of the opticalinterference filter. Specifically, the cumulative reflectance phasedispersion is chosen such that only the desired frequencies of light aretransmitted. Light passing through the optical interference filter isdetected and monitored via a photodetector. Optionally, the devices andmethods of the present invention may include a feedback meansoperationally coupled to the optical source and photodetector. Suchmeans feedback means are well known in the art and may be configured toprovide a means of maintaining a constant fraction of light from thelight source having a wavelength within the transmission bands of theetalon filter.

[0040] In another exemplary embodiment, the present invention comprisesa Gires-Tournois etalon filter (GT etalon) having substantiallyindependent, selectable free spectral range and resonance frequency. TheGT etalon filter of the present invention is particularly useful inwavelength division multiplexing applications, interleaver devices,deinterleaver devices and chromatic dispersion compensators. A preferredembodiment comprises a first partially reflective reflector comprising afirst sequence of thin dielectric layers and a second highly reflectivereflector comprising a second sequence of dielectric layers. A resonancecavity is formed between the two reflectors, which may be an air gapcavity or a dielectric layer cavity. The first and second sequences arecomprised of alternating high refractive index layers and low refractivelayers and one or more absentee layers. Such absentee layers may bepositioned in the first sequence, second sequence or both to provide fora selected cumulative reflectance phase dispersion of the GT etalon.Specifically, the number, position and/or composition of the absenteelayers are chosen to provide a substantially independent, selectablefree spectral range and resonance frequency.

[0041] The GT etalon filter of the present invention operates bymultiple-beam interference of light reflected and transmitted by firstpartial reflective reflector and second highly reflective reflector.Specifically, an incident light beam directed on to the GT etalon filterundergoes partial reflection and partial transmission upon interactionthe first reflector. The transmitted portion propagates through theresonance cavity and substantially all the light is reflected by thesecond reflector. The plurality of beams formed undergoes constructiveor destructive interference as they propagate in GT etalon. Importantly,the beams also interact with one or more absentee layers, which increasethe cumulative reflectance phase dispersion experienced. Preferredincreases in cumulative reflectance phase dispersion in the GT etalonfilter of the present invention range from about 1.0×10⁻⁶ rad/GHz toabout 1.0×10⁻³ rad/GHz. More preferred increases in cumulativereflectance phase dispersion range from about 1.0×10⁻⁶ rad/GHz to about5.0×10³¹ ⁵ rad/GHz. In a preferred embodiment, the cumulativereflectance phase dispersion is selected to provide a selected phasedispersion, which establishes the transmission properties of the etalonfilter.

[0042] The present invention includes methods for designing FP etalonfilters with substantially independent selected resonance frequency andfree spectral range. A preferred method of designing FP etalon opticalfilters with substantially independent selected resonance frequency andfree spectral range comprises: (1) entering into a suitably-programmedcomputer a starting structure comprising a first reflector comprising afirst sequence of thin dielectric films comprising alternating high andlow refractive index layers and a second reflector comprising a secondsequence of thin dielectric films comprising alternating high and lowrefractive index layers positioned a selected optical path length fromthe first reflector, (2) calculating the resonance frequencies and freespectral range associated with the starting structure, (3) adding one ormore absentee layers to the first sequence of thin dielectric layers,second sequence of dielectric layers or both creating a first modifiedstructure (4) calculating the cumulative reflectance phase dispersion,resonance frequencies and free spectral range associated with the firstmodified structure (5) modifying the position, optical thickness orcomposition of the absentee layer in the first sequence of thindielectric films and/or adding additional absentee layers to eitherfirst or second sequence of thin dielectric films, thereby creating anew modified structure, (4) calculating the cumulative reflectance phasedispersion, resonance frequencies and free spectral range associatedwith the new modified structure, (5) optimizing the composition of thenew modified structure to provide the desired resonance frequencies andfree spectral range.

[0043] The invention is further illustrated by the followingdescription, examples and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044]FIG. 1 is a schematic drawing of a FP etalon filter of the presentinvention having an air gap resonance cavity.

[0045]FIG. 2 is a schematic drawing of a FP etalon filter of the presentinvention having a dielectric layer resonance cavity.

[0046]FIG. 3 is a schematic drawing of a device employing the opticalinterference filter of the present invention for monitoring thefrequency of an optical source. FIG. 3 also illustrates a device fortuning the frequency of an optical source to a selected outputfrequency.

[0047]FIG. 4 shows a transmission spectrum of a FP etalon filter of thepresent invention (A). Also shown are the center frequencies of thetransmission channels of the International Telecommunications Unionfrequency standard (B). As illustrated in FIG. 4, the transmission bandsof the FP etalon filter are frequency matched to the transmissionchannels of the frequency standard to provide an intersection point onthe highly sloped region of the leading edge of the transmission band.

[0048]FIG. 5 is a comparison of the transmission spectra of the etalonfilter of the present invention and two prior art designs. Etalonresonance frequencies are depicted as solid lines and the centerfrequencies of the ITU frequency standard are shown as dashed lines.FIG. 5A shows the resonance frequencies of a prior art etalon designhaving a center resonance frequency of the reflector selected to matchthe transmission channels of the ITU grid. FIG. 5B shows the resonancefrequencies of a prior art etalon having a free spectral range selectedto match the transmission channels of the ITU grid. FIG. 5C shows thetransmission spectrum of the etalon optical filter of the presentinvention. FIG. 5C demonstrates that the etalon design of the presentinvention is capable of precise frequency matching to a plurality ofcenter frequencies of the ITU frequency standard.

DETAILED DESCRIPTION OF THE INVENTION

[0049] Referring to the drawings, like numerals indicate like elementsand the same number appearing in more than one drawing refers to thesame element. In addition, hereinafter, the following definitions apply:

[0050] “Thin dielectric layer” refers to a thin film comprising acoating of atoms, molecules or ions or mixtures thereof. Dielectriclayers useable in the present invention may comprise a single-layer or aplurality of thin dielectric layers. Thin dielectric layers useable inthe present invention may have either a homogeneous composition or aheterogeneous composition and may comprise a single phase or a pluralityof phases. In a preferred embodiment, reference to dielectric layers inthe present invention includes but is not limited to metal oxide thinfilms. Metal oxides useable in the present invention include but are notlimited to silica, Ta₂O₅, SiO₂, HfO₂, TiO₂, MgF₂, CaF₂, Nb₂O₅, glass ormixtures of these metal oxides. Dielectric layers of the presentinvention may also be composed of metal thin films such as Si layers.Dielectric layers of the present invention may be any size, shape,thickness or optical thickness. Thickness may be defined absolutely orrelative to the center resonance frequency of either reflector. Forexample, dielectric layers are commonly referred to as ¼ and ½ layersindicating an optical thickness approximately equal to the indicatedfraction of the wavelength of light corresponding to the centerresonance frequency of the reflectors comprising the filter. In apreferred embodiment, ¼ and ½ wave layers include but are not limited tothin dielectric layers having a optical thickness within 10% of ¼ or ½of the wavelength of light corresponding to the center resonancefrequency of the reflector. In a more preferred embodiment, ¼ and ½ wavelayers include but are not limited to thin dielectric layers having aoptical thickness within 5% of ¼ or ½ of the wavelength of lightcorresponding to the center resonance frequency of the reflector. Inaddition, dielectric layers of the present invention include layers thatare not ¼ or ½ wave layers. Embodiments having dielectric layers thatare not ¼ or ½ layers are useful for generating reflectors having anyarbitrary reflectivity. For example, dielectric layers of the presentinvention may have optical thicknesses that are less than or greaterthan ¼ wave layers. Preferred absolute thickness ranges from 5 nm-5000nm. More preferred absolute thickness range from 25 nm to 1500 nm.

[0051] “Resonance frequency” refers to the maximum frequency of atransmission band of an optical interference filter. For example, theresonance frequency of a Fabry-Perot filter with identical reflectors isgiven by the expression:$v_{m} = {\frac{c}{{2{nL}} + {\frac{c}{2\pi}\left( {\alpha_{1} + \alpha_{2}} \right)}}\left( {m - \frac{{\Phi_{1}\left( v_{c} \right)} - {\alpha_{1}v_{c}}}{2\pi} - \frac{{\Phi_{2}\left( v_{c} \right)} - {\alpha_{2}v_{c}}}{2\pi}} \right)}$

[0052] where ν is the resonance frequency, m is the order, Φ₁ is thereflection phase of the first reflector, and Φ₂ is the reflection phaseof the second reflector. ν_(c) is the center resonance frequency of eachreflector, which may be selected from a distribution of etalon resonancefrequencies positioned about the reflectance maximum of eitherreflector. Resonance frequency is related to the transmissionwavelength, which is the wavelength of maximum transmission, by theexpression: $\lambda = \frac{c}{v}$

[0053] where ν is frequency, λ is wavelength and c is the speed of lightin vacuum. Fabry Perot etalon filters of the present invention havetransmission spectra characterized by multiple transmission bands,resonance frequencies and transmission wavelengths. Optical interferencefilters of the present invention have a resonance frequency that isselectable, substantially independent of the free spectral range. In apreferred embodiment, resonance frequency is substantially independentfrom the free spectral range such that a 1% change in free spectralrange due to the presence of one or more absentee layers results in lessthan a 0.01% change in resonance frequency. In a more preferredembodiment, resonance frequency is substantially independent from thefree spectral range such that a 0.05% change in free spectral range dueto the presence of one or more absentee layers results in less than a0.00001% change in resonance frequency.

[0054] “Reflectors” refer generally to devices, device components andmaterials exhibiting reflectivity. Reflectors of the present inventioninclude partially reflective reflectors and reflectors that reflectsubstantially all incident light.

[0055] The reflectance of reflectors useable in the present inventionrange from about 5% to about 100%. Preferred reflectors of the presentinvention comprise single-layer or multilayer coatings with alternatinghigh and low indices of refraction layers. In a more preferredembodiment, the reflector of the present invention comprises thindielectric film sequences of alternating high and low indices ofrefraction. The terms “high” and “low” indices of refraction are definedrelative to one another. Accordingly, a “high” index of refraction isone larger than a “low” index of refraction and a “low” index ofrefraction is one smaller than a “high” index of refraction.

[0056] “Absentee layer” refers to a thin dielectric layer that exhibitssubstantially no reflectance at a selected wavelength but does affectthe reflectance phase dispersion of a sequence of thin dielectriclayers. In a preferred embodiment, an absentee layer has a reflectanceof less than 5%. In a more preferred embodiment, an absentee layer has areflectance of less than 1%. Absentee layers of the present inventionincrease the reflectance phase dispersion associated with a sequence ofthin dielectric layers. Preferred increases in cumulative reflectancephase dispersion attributable to the presence of one or more absenteelayers range from about 1.0×10⁻⁶ rad/GHz to about 1.0×10⁻³ rad/GHz. Morepreferred increases in cumulative reflectance phase dispersionattributable to the presence of one or more absentee layers range fromabout 1.0×10⁻⁶ rad/GHz to about 5.0×10⁻⁵ rad/GHz. Absentee layer opticalthickness may be defined absolutely or relative to the wavelength oflight corresponding to the center resonance frequency of eitherreflector. For example, absentee layer layers may have an opticalthickness of about ½ of the wavelength of light corresponding to thecenter resonance frequency of the reflector. In a preferred embodiment,absentee layers include thin dielectric layers having an opticalthickness within 10% of ½ of the wavelength of light corresponding tothe center resonance frequency of the reflector. In a more preferredembodiment, absentee layers have an optical thickness within 5% of ½ ofthe wavelength of light corresponding to the center resonance frequencyof the reflector. Preferred absolute thickness ranges from 5 nm-5000 nm.More preferred absolute thickness range from 50 nm to 1500 nm. Absenteelayers of the present invention may have the same composition as eitherhigh refractive index layers or low refractive index layers.Alternatively, absentee layers may have a composition different thanthat of the high refractive index layers or low refractive index layers.Absentee layers of the present invention include but are not limited todielectric layers composed of silica, Ta₂O₅, SiO₂, HfO₂, TiO₂, MgF₂,CaF₂, Nb₂O₅, glass, and Si.

[0057] “Reflectance phase dispersion” is an optical property thatcharacterizes the variation of the reflection phase shift at a reflectorsurface with the frequency of incident radiation. In a preferredembodiment, the reflectors of the present invention exhibit reflectancephase dispersion that may be approximated as a linear relationshipbetween the reflection phase shift at the virtual mirror surface andfrequency. This approximation may be expressed in terms of thereflectance phase dispersion by the following equation:Φ(v) = Φ(v_(c)) + α(v − v_(c))

[0058] where Φ(ν) is the reflection phase shift at the mirror surface,ν_(c) is the center resonance frequency of the reflector and α is thereflectance phase dispersion. In a preferred embodiment, the reflectionphase shift approximately linear with respect to frequency over thedesired frequency range. In preferred embodiments relating to thetelecommunications field, the reflection phase shift is approximatelylinear with respect to frequency over the frequency range of about 189THz to about 196 THz. The cumulative reflectance phase dispersion of anetalon optical filter of the present invention reflects the sum of thereflectance phase dispersion associated with each reflector:

α_(cumulative)=α₁+α₂

[0059] where α₁ is the reflectance phase dispersion of the firstreflector and α₂ is the reflectance phase shift of the second reflector.In a preferred etalon filter of the present invention, the cumulativereflectance phase dispersion is selected by adjusting the position andnumber of absentee layers in a sequence of dielectric layers comprisingat least one reflector.

[0060] “Free spectral range” is an optical property that characterizesthe spacing of transmission bands of an etalon filter. Specifically,free spectral range is a quantitative measure of the frequency spacingbetween successive transmission or phase maxima. Etalon filters of thepresent invention have a free spectral range that is selectable,substantially independent of the resonance frequency. Etalon filters ofthe present invention may have a substantially periodic free spectralrange, wherein the transmission bands are approximately equally spacedfrom each other with respect to frequency. In a preferred embodiment,the frequencies of substantially periodic transmission bands do notdeviate by more than 1% of the average spacing between transmissionbands. In a more preferred embodiment, the position of substantiallyperiodic transmission bands do not deviate by more than 0.1% of theaverage spacing between transmission bands. Alternatively, etalonfilters of the present invention may have an aperiodic spacing, whereinthe transmission bands are not equally spaced from each other withrespect to frequency.

[0061] “Bandwidth” refers to the property of optical filters related tothe distribution of wavelengths of light transmitted by a giventransmission band. Specifically, bandwidth is defined as the full widthat half maximum of a given transmission band. In a preferred embodiment,the bandwidth of the optical interference filters of the presentinvention is selected from the range of about 100 MHz to about 100 GHz.

[0062] “Frequency standard” refers to one or more selected frequenciesthat comprise an optical system. For example, a frequency standard maycomprise the transmission channels of a telecommunication system, suchas the ITU frequency grid. Transmission channels may comprise a singlefrequency or a range of frequencies. Frequency standard may also referto the emission lines of a given optical source, such as the modes of alaser or photoluminescent emitter.

[0063] “Transmission band” refers to a distribution of wavelengths,centered about a maximum transmission wavelength, which is transmittedby an interference filter. Interference filters of the present inventionare characterized by one or more transmission bands.

[0064] “Frequency matching” refers to a method of aligning one or moretransmission bands of an interference filter to overlap one or morefrequencies of a frequency standard. Interference filters of the presentinvention may be frequency matched to a selected frequency standard byselection of the number, position and composition of absentee layers ina sequence of dielectric layers comprising at least one reflector. FPetalon filters of the present invention are capable of being frequencymatched to any frequency standard, preferably to the ITU frequency grid.In a preferred embodiment, frequency matching refers to aligning thecenter frequencies of the transmission channels of a selected frequencystandard to a highly sloped region of the transmission band, such as theregion around approximately the half maximum of each etalon transmissionband. Such preferred alignment is particularly useful for wavelengthmonitoring and tuning applications because the slope of the transmissionband at the half maximum is large, and, thus, deviations from the centerfrequency of the transmission channel result in a large change inpercentage transmittance.

[0065] “Resonance cavity” refers to the space located between parallelreflectors of an optical interference filter. In a preferred embodiment,the resonance cavity of the present invention comprises a dielectriccavity layer positioned directly adjacent to the internal side of eachreflector. In a more preferred embodiment, the resonance cavitycomprises a metal oxide cavity layer including but not limited to fusedsilica, Ta₂O₅, SiO₂, HfO₂, TiO₂, MgF₂, CaF₂, Nb₂O₅, and glass.Alternatively, the resonance cavity of the present invention may be anair gap cavity. In this embodiment, the air gap may be substantiallyevacuated or be filled with a selected pressure of one or morenoncorrosive gas. Further, the air gap cavity may have a fixed, selectedrefractive index or a refractive index that is tunable. Resonancecavities of the present invention may have a fixed, selected opticalpath length, for a given angle of incidence. Alternatively, resonancecavities of the present invention may have tunable optical path length.Preferred optical path lengths are selected from the range of about 100nm to about 10 mm. More preferred optical path lengths are selected fromthe range of about 0.5 mm to about 5 mm.

[0066] “Ultra flat” refers to an extent of surface irregularity of agiven optical surface with a deviation from average surface positionapproximately on the order of the light impinging on the surface. Thespatial frequency of an ultra flat optical surface is smaller than thefrequency of light impinging on the surface. In a preferred embodiment,ultra flat surfaces of the present invention have deviations fromaverage surface position of less than 50 nm.

[0067] “Ultra smooth” refers to an extent of surface irregularity of agiven optical surface wherein the lateral distance between prominentsurface features is substantially smaller than the wavelength of lightimpinging on the surface. The spatial frequency of an ultra smoothoptical surface is greater than the frequency of light impinging on thesurface. In a preferred embodiment, ultra smooth surfaces of the presentinvention have deviations from average surface position of less than 1angstrom.

[0068] “Parallel” refers to a geometry in which two surfaces areequidistant from each other at all points and have the same direction orcurvature. Substantially parallel refers to a geometry in which alldeviations from absolute parallelism are less than 0.05 degree. In apreferred embodiment, the reflectors of the present invention arelocated in substantially parallel planes with respect to one another.

[0069] “Optical thickness” refers to the product of the thickness andthe refractive index of a layer and may be express by the equation:

optical thickness=(L)(n)

[0070] where L is the physical thickness and n is the refractive index.

[0071] “Center resonance frequency of a reflector” refers to a resonancefrequency of an etalon optical filter that is chosen from a distributionof resonance frequencies that are positioned about the reflectancemaximum of the first reflector, the second reflector or both. In apreferred embodiment of the present invention, the center resonancefrequency is within 2% of the reflectance maximum of the firstreflector, second reflector or both. In a more preferred embodiment ofthe present invention, the center resonance frequency is within 1% ofthe reflectance maximum of the first reflector, second reflector orboth.

[0072] This invention provides optical interference filters and methodsof using optical interference filters. In particular, the presentinvention provides optical interference filters with substantiallyindependent, selectable resonance frequency and free spectral range.

[0073]FIG. 1 illustrates an exemplary embodiment of the opticalinterference filter of the present invention having a resonance cavitycomprising an air gap cavity. The illustrated interference filter (100)comprises a first reflector (110) and second reflector (120) in opticalcommunication with each other and positioned to intersect propagationaxis (130). First reflector (110) has an external end (140) and aninternal end (150) and second reflector (120) has an external end (160)and an internal end (170). The internal end of second reflector (120) ispositioned a selected optical path length (180) from the internal end(150) of first reflector (110) and both reflectors are located insubstantially parallel planes with respect to each other.

[0074] First reflector (110) comprises a first sequence of thindielectric layers (190) on substrate (220). First sequence of thindielectric layers (190) comprises alternating high refractive indexlayers (200) and low refractive index layers (210). In addition, firstreflector (190) comprises absentee layer (230) positioned within firstsequence of thin dielectric layers (190). Optionally, first reflector(190) may comprise a plurality of absentee layers positioned withinfirst sequence of thin dielectric layers (190). Second reflector (120)comprises a second sequence of thin dielectric layers (235) on substrate(240). Second sequence of thin dielectric layers (235) comprisesalternating high refractive index layers (200) and low refractive indexlayers (210). Optionally, second reflector (120) may comprise at leastone absentee layer positioned within the second sequence of thindielectric layers (235). Resonance cavity (250) having selected opticalpath (180) for a given angle of incidence is formed between firstreflector (110) and second reflector (120) and is positioned alongpropagation axis (130). Specifically, resonance cavity (250) is an airgap cavity and occupies the space between substrate (220) and substrate(240). First reflector (110), second reflector (120) and resonancecavity (250) are oriented in a manner such that they are all in opticalcommunication with each other.

[0075] In a preferred embodiment, resonance cavity (250) has asubstantially constant optical path length for a given angle ofincidence. To achieve a highly stable optical path length, firstreflector (110) and second reflector (120) may be kinematically mountedin holder or spacer. In the embodiment depicted in FIG. 1, firstreflector (110) and second reflector (120) are held in place by spacer(255) and end plates (258). Such methods of kinematically mountingreflectors are well known in the art. Resonance cavity (250) may besubstantially a vacuum or may be composed of a selected pressure of oneor more gases. The composition of gases in resonance cavity (250)determines the refractive index and the thermal expansion coefficient ofthe cavity, which in turn influences the transmission characteristics ofthe filter. Any gas or combination of noncorrosive gases may be used inthe resonance cavity of the present invention including but not limitedto O₂, N₂, CO₂, SF₆, NF₃, CF₄ and C₂F₆.

[0076]FIG. 2 illustrates another exemplary embodiment of the opticalinterference filter of the present invention having a resonance cavitycomprising a dielectric cavity layer. The illustrated opticalinterference filter (400) comprises a dielectric cavity layer (410) ofselected optical path length (420) positioned along propagation axis(430). Dielectric cavity has a first end (440) and a second end (450)that intersects propagation axis (430). First reflector (460) has anexternal end (480) and an internal end (490) and is operationallycoupled to first end (440) of dielectric cavity layer (410). Secondreflector (470) has an external end (500) and an internal end (510) andis operationally coupled to second end (450) of dielectric cavity layer(410). First reflector (460) and second reflector (470) are in opticalcommunication with dielectrical cavity layer (410) and are located insubstantially parallel planes with respect to each other.

[0077] First reflector (460) comprises a first sequence of thindielectric layers (520) comprising alternating high refractive indexlayers (530) and low refractive index layers (540). In addition, firstreflector (460) comprises absentee layer (550) positioned within firstsequence of thin dielectric layers (520). Optionally, first reflector(460) may comprise a plurality of absentee layers positioned withinfirst sequence of thin dielectric layers (520). Second reflector (470)comprises a second sequence of thin dielectric layers (560) comprisingalternating high refractive index layers (530) and low refractive indexlayers (540). Optionally, second reflector (470) may comprise one ormore an absentee layer positioned within the second sequence of thindielectric layers (560).

[0078] Dielectric cavity layer (410) may be any dielectric materialincluding but not limited to glass, fused silica, quartz, sapphire,germanium, zinc selenide, Ta₂O₅, SiO₂, HfO₂, TiO₂, MgF₂, CaF₂ and Nb₂O₅.Use of low expansion materials is preferred to achieve a substantiallyconstant and stable optical path length as a function of temperature. Ina preferred embodiment, the dielectric cavity layer has ultra smooth andultra flat first end (440) and a second end (450).

[0079] First and second sequences of thin dielectric films may compriseany number of high and low refractive index pairs and any number ofabsentee layers. In a preferred embodiment, first and second sequenceseach comprise less than 10 high and low refractive index pairs. In anexemplary embodiment, high refractive index layers and low refractiveindex layers are metal oxide layers that are deposited on to a fusedsilica substrate. Preferably, high refractive index layers and lowrefractive index layers have an optical thickness equal to about ¼ thewavelength of light corresponding to center resonance frequency ofeither reflector and are made of Ta₂O₅ (refractive index of 2.025) andSiO₂ (refractive index of 1.445), respectively. In a preferredembodiment, fused silica substrate has a thickness equal to about 2 mm.In an exemplary embodiment, absentee layer is a metal oxide layer andhas an optical thickness equal to about ½ the wavelength of lightcorresponding to the center resonance frequency of the reflector. In apreferred embodiment, dielectric layers and substrates of the presentinvention have ultra smooth and ultra flat surfaces.

[0080] In order to produce thin film coatings with any arbitraryreflectivity, it is commonly necessary to use high refractive indexlayers and low refractive index layers that do not consist of quarterwave layers. Such uses of high refractive index layers and lowrefractive index layers that do not consist of quarter wave layers arewell known in the art of thin film optical coatings. Specifically, thelayer thicknesses of high refractive index layers and low refractiveindex layers are adjusted to achieve the desired reflectivity and thecumulative phase dispersion is selected by addition of absentee layers.Often, this adjustment results in high refractive index layers and lowrefractive index layers that deviate from ¼ of the wavelength of lightcorresponding to center resonance frequency of either reflector.Further, it is commonly necessary to further adjust the layerthicknesses of high refractive index layers and low refractive indexlayers to maintain the desired reflectivity with the addition ofabsentee layers.

[0081] Absentee layer may be in any position within first or secondsequence of thin dielectric layers. Positioning absentee layer close tothe external end of the first reflector increases the reflectance phasedispersion experienced substantially more than positioning absenteelayer close to internal end of the first reflector. Positioning absenteelayer close to internal end of the second reflector increases thereflectance phase dispersion experienced substantially more thanpositioning absentee layer close to external end of the secondreflector. Inclusion of a plurality of absentee layers in first sequenceof thin dielectric films, second sequence of dielectric films or bothfurther increases the cumulative reflectance phase dispersion. Forexample, current etalon designs are not capable of providing an opticalinterference filter with a free spectral range of 50.00 GHz and aresonance frequency at 194000 GHz. Prior art etalons having a resonancecavity optical path length of 2.997 mm are capable of achieving a freespectral range of 50.000 GHz. This cavity length, however, results in aresonance frequency of 194008.2 GHz. This deviation from the desiredresonance frequency is substantial and results from the relatively smallreflectance phase dispersion of prior art etalon reflectors. Suchreflectors typically have cumulative reflectance phase dispersions ofabout 2×10⁻⁵ rad/GHz. Addition of absentee layers to the dielectric thinfilms comprising the etalon reflectors, however, can substantiallyincrease the reflectance phase dispersion and compensate for thedeviation from the desired resonance frequency. For example, increasingthe reflectance phase dispersion of each reflector to 3.24×10⁻⁵ rad/GHz,results in an etalon having a free spectral range of 50.000 GHz and aresonance frequency of 194000.0 GHz.

[0082] The number of absentee layers in first sequence, second sequenceor both, the composition of absentee layers and the position of theabsentee layers of a given optical thickness, composition and positionin the first sequence, second sequence or both is selected to provide aselected cumulative reflectance phase dispersion. Computation of theincrease in reflectance phase dispersion caused by the inclusion of oneor more absentee layers may be performed by commercially availablesoftware, preferably Mcleod by Thin Film Center. Alternatively,determination of an absentee layer configuration necessary to achieve adesired free spectral range and resonance frequency may be determinedempirically.

[0083] Sequences of dielectric layers and dielectric cavity layersuseable in the present invention may be made by deposition and/orbonding techniques well known in the art of optical engineeringincluding but not limited to vapor deposition, chemical deposition,sputtering methods, optical contact techniques and the use of opticalcement. Sequences of dielectric layers may be flat or slightly wedgedshaped. In a preferred embodiment, each dielectric layer has an ultrasmooth surface and a substantially uniform composition. In anotherpreferred embodiment, the sequence of dielectric layer includes anantireflection layer, protective layer or both on its exterior end.

[0084] During operation as an optical filter, incident light having aselected angle of incidence is directed through the optical interferencefilter, wherein it interacts with the first reflector. A portion of theincident beam is reflected and the transmitted portion propagatesthrough resonance cavity and interacts with the second reflector. Thelight undergoes partial reflection upon every interaction with first andsecond reflectors and, thus, multiple beams are formed. Specifically,light undergoes partial reflection upon interaction with each highrefractive index—low refractive index pair and also experiences aselected reflectance phase dispersion upon interaction with absenteelayers present in the sequences of thin dielectric films. The multiplereflections cause interference between transmitted and reflected beams.The interference observed is either constructive or destructivedepending on the wavelength of the incident light and the optical pathlength. Only light of a selected frequency corresponding to thetransmission bands is observed to exit the optical interference filter.In a preferred embodiment, the composition, refractive index and numberof absentee layers and the position of absentee layers in firstsequence, second sequence or both is selected to achieve the desiredtransmission wavelengths, resonance frequencies and free spectral rangeof the interference filter.

[0085] In an exemplary embodiment, the FP etalon of the presentinvention comprises a GT etalon filter. In this embodiment, the firstreflector comprises a first partially reflective reflector and thesecond reflector comprises a highly reflective reflector. A resonancecavity having a fixed optical path length is formed between first andsecond reflectors, which may comprise an air gap cavity or dielectriclayer cavity. In a preferred embodiment, first reflector is composed ofa first sequence of thin dielectric layers having a net reflectanceselected from the range of about 1% to about 70% and the secondreflector is composed of a second sequence of thin dielectric layershaving a net reflectance selected from the range of about 90% to about100%. One or more absentee layers are present within the firstdielelectric layer sequence, second dielectric layer sequence or both toprovide a selected increase in the cumulative reflectance phasedispersion of the GT etalon. Specifically, the position, number,refractive index, optical thickness and composition of the absenteelayers in the first and/or second sequence of dielectric layers isselected to provide the desired increase in cumulative reflectance phasedispersion. In a preferred embodiment, the increase in cumulativereflectance phase dispersion associated with the presence of one or moreabsentee layers is selected to provide a selected cumulative reflectancephase dispersion which establishes the desired free spectral range andresonance frequency of the etalon filter.

[0086] The GT etalon filter of the present invention operates bymultiple-beam interference of light reflected and transmitted by thefirst partially reflected reflector and second highly reflectivereflector. Specifically, an incident beam is partially reflected by thefirst reflector. The transmitted portion of the beam propagates throughthe resonance cavity and is substantially all reflected by the secondreflector. Thus, the reflected portion is directed back through theresonance cavity and interacts with the first reflector. The multiplebeams formed upon each interaction with the first partially reflectivereflector undergo constructive and/or destructive interference such thatonly desired frequencies of light exit the GT etalon. The GT etalonfilter of the present invention is useful for wavelength divisionmultiplexing applications. For example, the GT etalon of the presentinvention may comprise a device component integrated into an interleaveror deinterleaver device. Further, the GT etalon filter of the presentinvention is useful for compensating chromatic dispersion and maycomprise a device component integrated into a dispersion compensator.

[0087] The optical interference filters of the present invention may beused to filter any light source including but not limited to (1)continuous sources such as solid-state lasers, semiconductor lasers, gasphase lasers, helium-neon lasers, atomic and molecular discharge lampsand (2) pulsed sources such as pulsed gas phase lasers, pulsed ormodulated solid-state semiconductor lasers and pulsed lamps. The opticalpath length of the resonance cavities of the present invention may beselectively adjusted by variation of the angle of incidence of theincident beam. Accordingly, the resonance frequency of the interferencefilters of the present invention may be adjusted by selection of theangle of incidence of the incident light beam by techniques well knownin the art of etalon optical filtering.

[0088]FIG. 3 illustrates an exemplary embodiment of an opticalarrangement for monitoring the frequency and intensity of an opticalsource. The illustrated optical arrangement comprises a light source(600) oriented such that the light beam (605) generated propagates alonglight generation axis (610). Light beam (605) is directed on to firstbeam splitter (620), which is in optical communication with light source(600) and positioned a selected distance from light source (600) suchthat it intersects light generation axis (610). A portion of light beam(605) is reflected by first beam splitter (620) and directed along afirst light-monitoring axis (640). The reflected portion of light beam(605) is passed through optical interference filter (630), which is inoptical communication with beam splitter (620) and positioned alongfirst light monitoring axis (640). Optical interference filter (630) isdesigned and arranged, as discussed above, to only permit passage ofincident light corresponding to selected resonance frequencies separatedby a selected free spectra range. Light having a frequency correspondingto the transmission bands exits optical interference filter (630) and isdetected by detector (660), which is in optical communication with firstbeam splitter (620) and is positioned along first light monitoring axis(640). All other frequencies of light do not pass though opticalinterference filter (630), and, hence, are not detected. By techniqueswell known in the art, the signal from detector (660) is measured andstored by computer (700), which is operationally connected to detector(660). Alternatively, the signal from detector (660) may be sentdirectly to feedback circuit (670), which is operationally connected todetector (660). Accordingly, this embodiment allows for monitoring theintensity of light from light source (600) having a wavelength withinthe distribution of wavelengths comprising the transmission bands ofoptical interference filter (630).

[0089] Optionally, the optical arrangement illustrated in FIG. 3 maycomprise a second beam splitter (680) in optical communication withlight source (600). In the preferred embodiment depicted in FIG. 3,second beam splitter (680) is positioned to intersect light propagationaxis (610). Incident light from light source (600) is directed at secondbeam splitter (680) and a portion is reflected along a second lightbeam-monitoring axis (690) and is detected by a second detector (695),which is in optical communication with beam splitter (680) andpositioned along second light monitoring axis (690). By techniques wellknown in the art, the signal from detector (695) is measured and storedby computer (700), which is operationally connected to detector (695).Alternatively, the signal from detector (695) may be sent directly tofeedback circuit (670). This embodiment of the present invention allowsfor simultaneous monitoring of the total intensity of light from lightsource (600) and the intensity of light from light source (600) having awavelength within the distribution of wavelengths comprising thetransmission bands of optical interference filter (630).

[0090] Optionally, the optical arrangement illustrated in FIG. 3 maycomprise feedback circuit (670) operationally connected to light source(600), detector (660) and detector (695). Feedback circuit (670) mayalso be operationally connected to computer (700). In a preferredembodiment, feedback circuit (670) maintains a constant ratio of thesignal from detector (660) and the signal from detector (695).Comparison of the magnitude of the signal from detector (660) and thesignal from detector (695) provides a means of evaluating the proportionof the output of light source (600) corresponding to the distribution ofwavelengths comprising the transmission bands of the filter. In a morepreferred embodiment, feedback circuit (670) is a differentiatingcircuit that differentiates the signal from detector (660) and thesignal from detector (695) and creates an error signal to correct thewavelength to the etalon and reference channel crossing point. In aneven more preferred embodiment, the crossing point is located at ahighly sloped region of the etalon transmission band, thus, providing avery sensitive means of monitoring and frequency tuning the output oflight source (600). Feedback element (670) may also be configured tooptimize the intensity of light from light source (600) having awavelength within the distribution of wavelengths comprising thetransmission bands of optical interference filter (630). It should beunderstood to one of skill in the art that feedback circuits are but onemeans for tuning the frequency and intensity of optical source (600) andthat the present invention also includes other feedback means well knownin the art.

EXAMPLE 1 FP Etalon Filter Frequency Matched to the TransmissionChannels of the ITU Frequency Standard

[0091] The ability of the optical interference filter of the presentinvention to be frequency matched to the transmission channels of theITU transmission grid was evaluated and compared to etalon designs inthe prior art. The standard channel definition of the ITU frequencystandard provides for a 45 channel system over a wavelength range of1533 nm to 1565 nm with a uniform channel spacing of 100 GHz(approximately 0.8 nm). Therefore, it is a goal of the present inventionto design an optical interference filter with a 100 GHz free spectralrange and with resonance frequency positions matched to the ITUfrequency standard. Further, the spacing of resonance peaks must bematched to the desired reference spacing to a very high degree ofaccuracy, commonly 1-5 ppm, to ensure efficient and accurate signaling.Accordingly, an optimal etalon must have a free spectral Range of 100GHz, corresponding to a mirror spacing of approximately 1.5 mm. Further,a transmission resonance peak is desired to have a frequency of 193994GHz, so that the negative slope edge of the resonance crosses the ITUGrid point frequency of 194000 GHz at-3dB (50%) transmission with aslope of about 8.3%/GHz. The etalon was designed to have nominallyidentical partial reflectors with a reflectance of 69%.

[0092] The prior art etalon designed to frequency match the ITUtransmission channels as close as possible comprises reflectors composedof a quarter wave layers of alternating high and low refractive index asshown in Table 1. TABLE 1 Prior Art Reflector Design of Etalon FrequencyMatch to the ITU Frequency Grid Refractive Layer # Material IndexOptical Thickness Medium Air 1.000 1 Ta2O5 2.025 0.25 2 SiO2 1.445 0.253 Ta2O5 2.025 0.25 4 SiO2 1.445 0.25 5 Ta2O5 2.025 0.25 Substrate Fused1.444 Silica

[0093] The prior art reflector design illustrated in Table 1 has a valueof the reflectance phase dispersion of approximately 2.02×10⁻⁵radians/GHz. When the prior art etalon is angle tuned to have aresonance frequency at 193994 GHz, its free spectral range is 99.984GHz. This corresponds to a deviation of 0.014 GHz compared to thenominally desired 100.000 GHz. Across 50 channels this error accumulatesto 0.7 GHz, a significant deviation that substantially impedeswavelength division multiplexing applications.

[0094] An etalon of the present invention adds a SiO₂ absentee layer of0.5 waves optical thickness between layers 2 and 3 in the first andsecond sequence of thin dielectric layers. This reflector composition isshown in Table 2. TABLE 2 Mirror Design for Etalon Filter FrequencyMatch to the ITU Frequency Grid Refractive Layer # Material IndexOptical Thickness Medium Air 1.000 1 Ta2O5 2.025 0.25 2 SiO2 1.445 0.252a SiO2 1.445 0.50 3 Ta2O5 2.025 0.25 4 SiO2 1.445 0.25 5 Ta2O5 2.0250.25 Substrate Fused 1.444 Silica

[0095] The addition of the absentee layers in first and secondreflectors does not affect the reflectance significantly. The presenceof the absentee layers does, however, substantially increase thereflectance phase dispersion of each reflector to approximately3.14×10⁻⁵ radians/GHz. When the etalon of the present invention is angletuned to have a resonance frequency at 193994 GHz, its free spectralrange is 100.000 GHz. This matches the desired free spectral range tobetter than 0.5 MHz. Across 50 channels this error accumulates to atmost 0.025 GHz. Accordingly, the etalon design shown Table 2 satisfiesthe frequency constraints of the ITU frequency standard, within verynarrow tolerances.

[0096]FIG. 4 shows a transmission spectrum of the etalon described inthis example (A) with the center frequencies of the ITU transmissionchannels (B) indicated for comparison. As shown in FIG. 4, the centerfrequencies of the ITU transmission channels are positioned atapproximately the same frequency as the half maximum of the leading edgeof each etalon transmission band. This spectral overlap is beneficialbecause it permits very sensitive frequency monitoring when the etalonis used in wavelength discrimination applications. Each intersectionpoint of the etalon transmission band and center frequency of each ITUtransmission channel exhibits a very large slope. This alignment resultsin a large change in percentage transmittance when the frequency oflight propagating through the etalon deviates from the center frequencyof a given ITU transmission channel. Accordingly, the etalon reflectordesign set forth in Table 1 is particularly useful for frequencymonitoring and wavelength tuning applications.

[0097]FIGS. 5A, B and C show a comparison of the transmission spectra ofthe etalon filter of this example with two prior art etalon designs.Etalon resonance frequencies are depicted as solid lines and the centerfrequencies of the ITU grid are shown as dashed lines. FIG. 5demonstrates that etalons of the present invention are capable ofprecisely frequency matching optical signals to a plurality of centerfrequencies of the ITU frequency standard. Further, FIG. 5 demonstratesthat prior art etalon designs are able to precisely match, at best, onlyone transmission channel of the ITU frequency grid. FIG. 5A shows theresonance frequencies of a prior art etalon design having a centerresonance frequency of the reflector selected to match a selectedtransmission channel of the ITU grid. As shown in FIG. 5A, at n equal tozero (the center resonance frequency of the reflector) the overlapbetween etalon resonance frequency and the ITU frequency standard isvery good. Substantial deviations between resonance frequency and ITUfrequencies are evident, however, for all other orders of the etalon.FIG. 5B shows the resonance frequencies of a prior art etalon having afree spectral range selected to match the transmission channels of theITU grid. As shown in FIG. 5B, no overlap between etalon resonancefrequency and the ITU frequencies is observed. FIG. 5C shows thetransmission spectrum of the etalon optical filter of this example. Asshown in FIG. 5C, the resonance frequencies of the etalon of the presentexample substantially overlap the center frequencies of the ITU grid forall the orders shown. Accordingly, the etalon of the present example isvery useful for frequency matching applications involving the ITUfrequency standard. It should be apparent to persons of ordinary skillin the art that the etalon filters of the present invention may beaccurately frequency matched to any frequency substantially periodicstandard, not just the transmission channels of the ITU.

EXAMPLE 2 GT Etalon Filter Frequency Matched to the TransmissionChannels of the ITU Frequency Standard

[0098] A GT etalon of the present invention capable of frequencymatching to the transmission channels of the ITU frequency grid wasevaluated and compared to GT etalon designs in the prior art.Specifically, a GT etalon was designed to have a free spectral range aclose as possible to 50 GHz and a resonance frequency as close aspossible to a frequency of 194000 GHz. Further, the GT etalon wasdesigned to comprise a partial reflector with a reflectance ofapproximately 48% at one mirror and a high reflector with a reflectanceof 99.95%.

[0099] Prior art GT etalon designs with a free spectral as close aspossible to 50 GHz and a resonance frequency as close as possible to afrequency of 194000 GHz comprises a partially reflective reflector andhighly reflective reflector separated by a resonance cavity with anoptical path length of approximately 3.0 mm. Specifically, the partiallyreflective reflector comprises a first sequence of quarter wave layersof alternating high and low refractive indexes as shown in Table 3 andhas a reflectance phase dispersion of approximately 1.37×10⁻⁵radians/GHz. TABLE 3 Prior Art Partially Reflective Reflector Design forGT Etalon Frequency Matched to the ITU Frequency Grid Refractive Layer #Material Index Optical Thickness Medium Air 1.000 1 Ta2O5 2.025 0.25 2SiO2 1.445 0.25 3 Ta2O5 2.025 0.25 Substrate Fused 1.444 Silica

[0100] The highly reflective reflector of the prior art GT etalon designcomprises a second sequence of quarter wave layers of alternating highand low refractive indexes as shown in Table 4 and has a reflectancephase dispersion of approximately 2.89×10⁻⁵ radians/GHz. TABLE 4 PriorArt Highly Reflective Reflector Design for GT Etalon Frequency Matchedto the ITU Frequency Grid Refractive Layer # Material Index OpticalThickness Medium Air 1.000  1 Ta2O5 2.025 0.25  2 SiO2 1.445 0.25  3Ta2O5 2.025 0.25  4 SiO2 1.445 0.25  5 Ta2O5 2.025 0.25  6 SiO2 1.4450.25  7 Ta2O5 2.025 0.25  8 SiO2 1.445 0.25  9 Ta2O5 2.025 0.25 10 SiO21.445 0.25 11 Ta2O5 2.025 0.25 12 SiO2 1.445 0.25 13 Ta2O5 2.025 0.25 14SiO2 1.445 0.25 15 Ta2O5 2.025 0.25 16 SiO2 1.445 0.25 17 Ta2O5 2.0250.25 18 SiO2 1.445 0.25 19 Ta2O5 2.025 0.25 20 SiO2 1.445 0.25 21 Ta2O52.025 0.25 22 SiO2 1.445 0.25 23 Ta2O5 2.025 0.25 24 SiO2 1.445 0.25 25Ta2O5 2.025 0.25 Substrate Fused 1.444 Silica

[0101] When the prior art GT etalon of this composition is tuned toprovide a resonance frequency at 194000 GHz, its free spectral range is50.00845 GHz. This constitutes a substantial deviation of 0.00845 GHzfrom the desired free spectral range of 50.000 GHz Across 100 channelsthis error accumulates to 0.85 GHz, a significant deviation.

[0102] A GT etalon design of the present invention modifies thecomposition of the prior art GT etalon design by incorporatingadditional absentee layers in the second sequence of dielectric layerscomprising the highly reflective reflector. Specifically, a GT etalon ofthe present invention retains the first reflector compositionillustrated in Table 3. The inventive GT etalon design, however,modifies the composition of the highly reflective reflector by adding aTa₂O₅ layer of 0.5 wave optical thickness between layers 1 and 2, and asecond Ta₂O₅ layer of 0.5 wave optical thickness between layers 5 and 6.The modified highly reflective reflector design of the present inventionis shown in Table 5. TABLE 5 Highly Reflective Reflector Design for GTEtalon Frequency Matched to the ITU Frequency Grid Refractive Layer #Material Index Optical Thickness Medium Air 1.000  1 Ta2O5 2.025 0.25 1a Ta2O5 2.025 0.50  2 SiO2 1.445 0.25  3 Ta2O5 2.025 0.25  4 SiO21.445 0.25  5 Ta2O5 2.025 0.25  5a Ta2O5 2.025 0.50  6 SiO2 1.445 0.25 7 Ta2O5 2.025 0.25  8 SiO2 1.445 0.25  9 Ta2O5 2.025 0.25 10 SiO2 1.4450.25 11 Ta2O5 2.025 0.25 12 SiO2 1.445 0.25 13 Ta2O5 2.025 0.25 14 SiO21.445 0.25 15 Ta2O5 2.025 0.25 16 SiO2 1.445 0.25 17 Ta2O5 2.025 0.25 18SiO2 1.445 0.25 19 Ta2O5 2.025 0.25 20 SiO2 1.445 0.25 21 Ta2O5 2.0250.25 22 SiO2 1.445 0.25 23 Ta2O5 2.025 0.25 24 SiO2 1.445 0.25 25 Ta2O52.025 0.25 Substrate Fused 1.444 Silica

[0103] Although the reflector design illustrated in Table 5 retains areflectance of approximately 99.95%, it has a value of the reflectancephase dispersion of approximately 5.09×10⁻⁵ radians/GHz. When the GTetalon of the present invention is tuned to have a resonance frequencyat 194000 GHz, its free spectral range is 50.00005 GTz. This etalondesign matches the desired free spectral range to approximately 0.05MHz. Across 100 channels this error accumulates to at most 0.01 GTz, amuch less significant deviation than that observed in prior art GTetalons.

We claim:
 1. An optical interference filter comprising: a) a firstreflector, having an internal end and an external end, said reflectorcomprising a first sequence of thin dielectric layers comprisingalternating high and low indices of refraction layers and an absenteelayer; and b) a second reflector positioned a selected distance from theinternal end of said first reflector, having an internal end and anexternal end, said second reflector comprising a second sequence of thindielectric layers comprising alternating high and low indices ofrefraction layers, wherein said first reflector and said secondreflector are located in substantially parallel planes with respect toone another and thereby form a resonance cavity between the firstreflector and the second reflector; wherein the position of saidabsentee layer in said first sequence of thin dielectric layers,composition of said absentee layer or both is selected to provide aselected cumulative reflectance phase dispersion.
 2. The opticalinterference filter of claim 1 wherein the reflectance phase dispersionis selected to provide a substantially independent, selectable resonancefrequency and free spectral range of the optical interference filter. 3.The optical interference filter of claim 1 having a transmissionspectrum comprising substantially periodic transmission bands.
 4. Theoptical interference filter of claim 1 wherein the first sequence ofthin dielectric layers comprises a plurality of said absentee layers andwherein the number of said absentee layers, the position of saidabsentee layers in the first sequence of thin dielectric layers or bothare selected to provide a selected cumulative reflectance phasedispersion.
 5. The optical interference filter of claim 1 wherein saidsecond sequence of thin dielectric layers comprises an absentee layerwherein the position of said absentee layer in said second sequence ofthin dielectric layers is selected to provide a selected cumulativereflectance phase dispersion.
 6. The optical interference filter ofclaim 1 wherein the second sequence of thin dielectric layers comprisesa plurality of said absentee layers and wherein the number of saidabsentee layers, the position of absentee layers in the second sequenceof thin dielectric layers or both are selected to provide a selectedcumulative reflectance phase dispersion.
 7. The optical interferencefilter of claim 1 wherein said absentee layers are substantiallytransparent to light of wavelength equal to light having a frequencyequal to the selected resonance frequency.
 8. The optical interferencefilter of claim 1 wherein the cumulative reflectance phase dispersion isselected from the range of about 1×10⁻⁵ rad/GHz to about 10×10⁻⁵rad/GHz.
 9. The optical interference filter of claim 1 wherein thethickness of said absentee layer is determined by the equation:${T_{A} = {(m)\left( \frac{c}{2{nv}_{c}} \right)}};$

wherein T_(A) is the thickness of the absentee layer, m is a positiveinteger, n is the refractive index and ν_(c) is the center resonancefrequency of the first reflector, second reflector or both.
 10. Theoptical interference filter of claim 1 wherein the absentee layer is ametal oxide.
 11. The optical interference filter of claim 1 wherein theabsentee layer is selected from the group consisting of: a) glass; b)Ta₂O₅; c) SiO₂; d) HfO₂; e) TiO₂; f) MgF₂; g) CaF₂; h) Nb₂O₅; and i)quartz;
 12. The optical interference filter of claim 1 wherein saidresonance cavity comprises an air gap cavity.
 13. The opticalinterference filter of claim 12 wherein said first reflector comprises afirst substrate layer and said second reflector comprises a secondsubstrate layer.
 14. The optical interference filter of claim 2 whereinthe refractive index of said air gap cavity is selectively adjustable.15. The optical interference filter of claim 12 wherein said air gapcavity is filled with one or more gases.
 16. The optical interferencefilter of claim 1 wherein said resonance cavity comprises a dielectriccavity layer.
 17. The optical interference filter of claim 16 whereinsaid dielectric cavity layer is a metal oxide.
 18. The opticalinterference filter of claim 16 wherein said dielectric cavity layer isselected from the group consisting of: a) glass; b) fused silica; c)Ta₂O₅; d) SiO₂; e) HFO₂; f) TiO₂; g) MgF₂; h) CaF₂; i) Nb₂O₅; and j) Si.19. The optical interference filter of claim 1 wherein the optical pathlength of said resonance cavity is selected from the range of about 100nm to about 10 mm.
 20. The optical interference filter of claim 1 havinga finesse selected from the range of about 3 to about
 10. 21. Theoptical interference filter of claim 2 wherein the resonance frequencyis about 194000 GHz and the free spectra range is about 100 GHz.
 22. Theoptical interference filter of claim 2 wherein the resonance frequencyis about 194000 GHz and the free spectra range is about 50 GHz.
 23. Theoptical interference filter of claim 2 wherein the resonance frequencyis about 194000 GHz and the free spectra range is about 25 GHz.
 24. Theoptical interference filter of claim 2 wherein the resonance frequencyis about 194000 GHz and the free spectra range is about 200 GHz.
 25. Theoptical interference filter of claim 1 having a bandwidth selected fromthe range of about 100 MHz to about 100 GHz.
 26. The opticalinterference filter of claim 1 wherein the optical thickness of eachhigh index of refraction layer is about one quarter of the wavelength oflight corresponding to the center resonance frequency of the firstreflector, second reflector or both.
 27. The optical interference filterof claim 1 wherein the optical thickness of each low index of refractionlayer is about one quarter of the wavelength of light corresponding tothe center resonance frequency of the first reflector, second reflectoror both.
 28. The optical interference filter of claim 1 wherein theinternal and external ends of said first reflector and said secondreflector are ultra flat optical surfaces.
 29. The optical interferencefilter of claim 1 wherein the composition of the absentee layer is thesame as the composition of the high refractive index layers.
 30. Theoptical interference filter of claim 1 wherein the composition of theabsentee layer is the same as the composition of the low refractiveindex layers.
 31. The optical interference filter of claim 1 wherein thefirst reflector is slightly wedge shaped.
 32. The optical interferencefilter of claim 1 wherein the second reflector is slightly wedge shaped.33. The optical interference filter of claim 1 wherein the firstreflector and the second reflector have about the same reflectance. 34.The optical interference filter of claim 1 wherein the first reflectorand the second reflector have different reflectance.
 35. The opticalinterference filter of claim 1 comprising a fixed wavelength filter. 36.The optical interference filter of claim 1 comprising a tunablewavelength filter.
 37. The optical interference filter of claim 1wherein the external ends of said first reflector and said secondreflector have an antireflection coating.
 38. The optical interferencefilter of claim 2 having a resonance frequency and the free spectralrange frequency matched to a selected frequency standard.
 39. Theoptical interference filter of claim 38 wherein the selected frequencystandard is the International Telecommunication Union frequencystandard.
 40. The optical interference filter of claim 38 wherein theselected frequency standard comprise the emission lines of an opticalsource.
 41. The optical interference filter of claim 1 wherein theselected cumulative reflectance phase dispersion is substantially linearwith respect to frequency.
 42. The optical interference filter of claim1 wherein the selected cumulative reflectance phase dispersion issubstantially nonlinear with respect to frequency.
 43. An opticalinterference filter comprising a plurality of optical interferencefilters of claim 1 positioned to intersect a common propagation axis andlocated in substantially parallel planes with respect to one another.44. An optical interference filter comprising: a) a first partiallyreflective reflector, having an internal end and an external end, saidreflector comprising a first sequence of thin dielectric layerscomprising alternating high and low indices of refraction layers; and b)a second highly reflective reflector positioned a selected distance fromthe internal end of said first reflector, having an internal end and anexternal end, said second reflector comprising a second sequence of thindielectric layers comprising alternating high and low indices ofrefraction layers and an absentee layer, wherein said first reflectorand said second reflector are located in substantially parallel planeswith respect to one another and thereby form a resonance cavity betweenthe first reflector and the second reflector; wherein the position ofsaid absentee layer in said second sequence of thin dielectric layers,composition of said absentee layer or both are selected to provide aselected cumulative reflectance phase dispersion.
 45. The opticalinterference filter of claim 44 wherein the cumulative reflectance phasedispersion is selected to provide a substantially independent,selectable resonance frequency and free spectral range of the opticalinterference filter.
 46. The optical interference filter of claim 45having a transmission spectrum comprising substantially periodictransmission bands.
 47. The optical interference filter of claim 44wherein the second sequence of thin dielectric layers comprises aplurality of said absentee layers and wherein the number of saidabsentee layers, the position of said absentee layers in the secondsequence of thin dielectric layers or both are selected to provide aselected cumulative reflectance phase dispersion.
 48. The opticalinterference filter of claim 44 wherein said first sequence of thindielectric layers comprises an absentee layer wherein the position ofsaid absentee layer in said first sequence of thin dielectric layers isselected to provide a selected cumulative reflectance phase dispersion.49. The optical interference filter of claim 44 wherein the firstsequence of thin dielectric layers comprises a plurality of saidabsentee layers and wherein the number of said absentee layers, theposition of said absentee layers in the first sequence of thindielectric layers or both are selected to provide a selected cumulativereflectance phase dispersion.
 50. The optical interference filter ofclaim 44 wherein said second reflector has a reflectance greater than90%.
 51. The optical interference filter of claim 44 wherein said secondreflector reflects substantially all incident light.
 52. A method ofmonitoring the frequency of light generated by an optical sourcecomprising the steps: a) passing light from said optical source througha beam splitter; b) directing light reflected from said beam splitterthrough an optical interference filter, said optical interference filtercomprising a first reflector and a second reflector, said firstreflector having an internal end and an external end, and comprising afirst sequence of thin dielectric layers comprising alternating high andlow indices of refraction layers and an absentee layer, said secondreflector positioned a selected distance from the internal end of saidfirst reflector, having an internal end and an external end, said secondreflector comprising a second sequence of thin dielectric layerscomprising alternating high and low indices of refraction layers,wherein said first reflector and said second reflector are located insubstantially parallel planes with respect to one another and therebyform a resonance cavity between the first reflector and the secondreflector, wherein the position of said absentee layer in said firstsequence of thin dielectric layers, composition of said absentee layeror both are selected to provide a substantially independent, selectableresonance frequency and free spectral range of the optical interferencefilter; and c) detecting light passed through said second reflector witha photodetector.
 53. A method of tuning the frequency of light generatedby an optical source comprising the steps: a) passing light from saidoptical source through a first beam splitter; b) directing lightreflected from said first beam splitter through an optical interferencefilter, said optical interference filter comprising a first reflectorand a second reflector, said first reflector having an internal end andan external end, and comprising a first sequence of thin dielectriclayers comprising alternating high and low indices of refraction layersand an absentee layer, said second reflector positioned a selecteddistance from the internal end of said first reflector, having aninternal end and an external end, said second reflector comprising asecond sequence of thin dielectric layers comprising alternating highand low indices of refraction layers, wherein said first reflector andsaid second reflector are located in substantially parallel planes withrespect to one another and thereby form a resonance cavity between thefirst reflector and the second reflector, wherein the position of saidabsentee layer in said first sequence of thin dielectric layers,composition of said absentee layer or both are selected to provide asubstantially independent, selectable resonance frequency and freespectral range of the optical interference filter; c) detecting lightpassed through said second reflector with a first photodetector, whereina first signal is obtained; d) passing light from said optical sourcethrough a second beam splitter; e) detecting light reflected from secondreflector with a second photodetector, wherein a second signal isobtained; d) comparing the first signal and second signal and adjustingthe operating conditions of said optical source to maintain a constantratio of the first signal to second signal.